1. Field of the Invention
The invention relates to the cross-linking of cellulose fibers, more particularly to wood pulp fibers.
2. Prior Art
The cross-linking reaction imparts to the cellulose fibers -wood fibers or cotton linters- new characteristics, especially a greatly increased warter absorptivity.
The links between the anhydroglucose units of the cellulose chains hinder the inter-fiber bonds, impart stiffness to the fibers and increase the water absorption through capillarity.
Cross-linked fibers are useful in the preparation of napkins, pads and diapers, but also of sheet materials having improved bulk, softness as well as reduced tensile strength. If employed with a resinous binder, the modified fibers are particularly useful in the manufacture of non-woven characterized by their improved softness, bulk, caliper and absorbency.
The cross-linked fibers are well-known in the prior art: see, for example French patent No. 892,799 (Westfalische Zellstoff) and U.S. Pat. No. 2,010,635 (J. Kantorowicz), but the technical difficulties raised up numerous searches. See for example, U.S. Pat. No. 3,224,926 (C. J. Bernardin), U.S. Pat. No. 3,440,135 (R. Chung) and U.S. Pat. No. 3,700,549.
The U.S. Pat. Nos. 3,224,926 and 3,440,135 describe processes which require an impregnation step -with the cross-linking agent or with the catalyst- and a drying or storage step for periods of time up to 48 hours, a defiberizing step and a thermal treatment step.
The formaldehyde which is the less expensive cross-linking agent and fully effective at low levels (see U.S. Pat. No. 3,224,926 col. 2, ls. 23-27) is considered as less desirable because of its volatility (see U.S. Pat. No. 3,440,135 col. 7, ls. 55-57). The use of more expensive compounds and/or the too long aging or drying steps have prevented any wide scale commercial manufacture of the cross-linked fibers.
It is also well-known that paper or cardboard might be treated with formaldehyde [see for example U.S. Patent 1,816,973 (J. Kantorowicz), U.S. Pat. No. 3,264,054 (R. Reinhardt and al), U.S. Pat. No. 3,310,363 (J. Russel and al)]. All these processes are attempts in order to improve the physical properties --especially wet tensile strength-- of the sheets and do not teach how to treat individual cellulose fibers. The cross-linking of the cellulose is a double etherification of the primary alcohol groups of the anhydroglucose units with the cross-linking agent. Various compounds may be used: formaldehyde, polyoxymethylene, trioxane, aminoplasts, glyoxal, etc. The reaction is catalyzed by acids (Lewis acids, low molecular weight organic acids).
Other compounds like epichlorydrine or other expoxides require a basic catalyst.
An important feature of the cross-linking reaction is the thermal treatment: the cellulose is adversely affected by heat and acids. A balance between acid concentration and temperature has to be found.
The cross-linking reaction may occur in liquid or vapor phase.
Two ways have to be distinguished among the "liquid-phase" type processes:
low fiber concentration processes PA1 high fiber concentration processes.
They are obtained by bringing to the fibers the minimum amount of cross-linking agent or by removing the excess of reagents by squeezing an impregnated sheet.
Such methods allow a good distribution of the chemicals and swelling of the fibers, especially when water or phosphoric acid are used as solvents.
The reaction is intra lamellar and the processed fibers are quite different from those obtained with dried and flat fibers.
As a matter of fact, the water content of the reaction medium is very important: in order to obtain flat cross-linked fibers less than 18% water must be used. The cross-linking reaction is then a surface modificator and even after drying --as in the papermaking art-- the fibers remain unbonded. Less water content might be obtained with reactions in solvent medium: acetone, dioxane, acetic acid. In boiling acetone, the reaction time might last 30 seconds to 10 minutes. Following a method described in the French Patent No. 2,224,485:
______________________________________ Fibers Formaldehyde Hcl Water Acetone ______________________________________ Concentration 5 4 4.9 9.5 76.6 Reaction time: 10 minutes Water absorption value: 31 g/g. ______________________________________
However the use of such large amounts of acetone and the regeneration of the solvent-catalyst mixture prevent any industrial development of this process. The vapor phase reaction allows an excellent repartition of the reagents in the fibers, but the use of hydrochloric acid or other strong acid must be avoided. It is then necessary to use low molecular weight organic acids: formic, acetic acid. Reaction time is longer, the fibers are not damaged but the regeneration step and the acid losts are expensive. Water absorptivities between 27g/g and 29g/g are obtained.